Electric power steering apparatus

ABSTRACT

Provided is an electric power steering apparatus comprising a housing, a motor attached to this housing and transmitting an auxiliary steering force to a rotary shaft through a motor shaft, a worm formed on or fitted on the rotary shaft and having a gear portion formed of a metal or a resin, rolling bearings provided in the housing, disposed respectively in positions on both sides of the worm and rotatably supporting the rotary shaft, an output shaft transmitting a steering force for steering an axle and rotatably supported in a predetermined position of the housing, and a worm wheel formed on or fitted on the output shaft in a way that meshes with the worm and having a gear portion formed of the resin, wherein a pre-load mechanism for applying a pre-load acting towards the worm wheel is provided at a shaft end portion, distal from the motor, of the rotary shaft. The thus simply-constructed electric power steering apparatus is capable of eliminating an existence of a backlash and reducing tooth-butting noises without any decline of a power transmitting performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/491,564filed Apr. 5, 2004, which is a 371 of PCT/JP02/12651 filed Dec. 3, 2002.

TECHNICAL FIELD

The present invention relates generally to an electric power steeringapparatus including a worm deceleration mechanism, and more particularlyto an electric power steering apparatus contriving an improvement innoises caused by a backlash.

BACKGROUND ART

Generally, a play (backlash) is provided between gears in order toobtain a smooth operation in gear meshing.

A worm wheel deceleration mechanism of the electric power steeringapparatus also requires a proper backlash. Normally, an axis-to-axisdistance between a gear housing shaft of a housing and an output shaftof a motor is set to the same as an axis-to-axis distance (a worm gearworking radius plus a wheel gear working radius), and hence a backlashderived from a variety of working scatters in addition to the presetbacklash. This caused-by-the-scatter backlash gives a driver anuncomfortable feeling due to tooth-butting noises when a vehicle travelson a rough road, resulting in a decline of value of a commercialproduct.

A countermeasure therefor was that the tooth-butting noises are reducedby setting the backlash as small as possible while raising a gearaccuracy, or by a method, as disclosed in Japanese Patent ApplicationLaid-Open No. 11-43062, of absorbing vibrations in a way that providesan elastic body between a worm shaft and a bearing thereof, and soforth.

Moreover, Japanese Patent Application Laid-Open No. 10-281235 disclosesa power transmission apparatus, wherein the worm shaft is supportedthrough the bearing in a shaft hole taking an elliptic shape that isformed in a worm shaft housing portion and is eccentric on the side ofthe worm wheel, and O-ring shaped elastic member is provided in acircular groove formed concentrically in an inner peripheral surface ofthe shaft hole, this elastic member biases the bearing (the worm shaft)towards the worm wheel, thereby eliminating the backlash.

Further, when assembling the worm shaft and the worm wheel, there mightoccurs a dimensional error between the worm, the worm shaft, a bearingportion supporting the worm shaft, the worm wheel and a steering shaftfor supporting the worm wheel, etc.. It follows that the backlash iscaused at a comparatively large rate due to this dimensional error afterbeing assembled. It was therefore required that the parts be assembledseparately according to the accuracy thereof. Further, if a higheroutput of an auxiliary steering force advances as seen over the recentyears, this results in an increase in abrasions of teeth of the worm andof the worm wheel, and a drawback comes to appear, wherein theoccurrence of the backlash can not be avoided.

A known method for preventing gear butting noises derived from thesecauses is a method of eliminating the backlash by applying a pre-load tothe worm towards the worm wheel. For example, as disclosed in JapanesePatent Application Laid-Open Nos. 2001-322554 and 2001-108025, there areknown methods of generating a pre-load force by causing a deformation ofan elastic body provided between an outer ring of the bearing providedat a side end of the worm and a gear housing.

Of the conventional electric power steering apparatuses disclosed in theformer Publications, the apparatus disclosed in Japanese PatentApplication Laid-Open No. 10-281235 has a drawback that the worm shafthousing portion is easy to abrade when the bearing moves and a problemthat the worm shaft is easy to cause an axis deviation with respect tothe motor shaft when the worm shaft is biased by the elastic member, andso on.

Generally, as far as there exists the backlash depending on a conditionof a rough road, a difference between inputs from a motor vehicle and soon, a problem is that the tooth-butting noises can not be completelymuffled, and there is a necessity of reducing the tooth-butting noisesfor every motor vehicle.

In the conventional electric power steering apparatuses disclosed in thelatter Publications, a problem is that the quantity of deformation ofthe elastic body is determined from an outside diameter of a rollingbearing that is determined based on an inside diameter of a housing anda load, the elastic body has no alternative but minutely deforms becauseof a restraint in terms of a space, besides a pre-load force for theworm largely changes due to a minute displacement of a worm end that iscaused by a scatter in working of the gear housing and by a deflectionin meshing, and it is therefore difficult to ensure the pre-load forceexpected.

If this pre-load force is too large, an operating force declines, whichbrings about deterioration in feeling when neutral of steering. Whereasif too small, the gear butting noises emit, and an essential purpose cannot be attained.

Thus, according to the prior art, even in a case where an axis-to-axisdistance between the worm and the worm wheel changed due to the gearabrasion, etc., the pre-load varies due to the minute displacement ofthe worm, and it was difficult to ensure the stable pre-load.

Moreover, the load and a rotational torque acting in radial directionsare applied to the elastic body provided on the outer periphery of thebearing whenever steered, and hence, as disclosed in Japanese PatentApplication Laid-Open No. 2001-270448, deterioration such as a fatigue,etc occurs in the elastic body according to a structure for makingflexural the elastic body serving as a slide bearing, and a problemarises from this deterioration, wherein the quantity of backlash rises,and the pre-load force decreases from a permanent deformation of theelastic body.

The load and the rotational torque acting in the radial directions havea great influence on the pre-load force given by the elastic body, withthe result that the quantity of deformation of the elastic bodyaugments. Therefore, if the axis-to-axis distance between the worm andthe worm wheel increases and if a meshing area between the gearsdecreases, there also arises a problem that a strength of the geardeclines this time.

An object of the present invention lies in providing a simply-structuredelectric power steering apparatus capable of improving the drawbacks tothe examples of the prior art described above, eliminating an existenceof the backlash and reducing tooth-butting noises without any decline ofa power transmitting performance.

DISCLOSURE OF THE INVENTION

To accomplish the above object, according to a first invention of thepresent invention, an electric power steering apparatus with a worm gearmechanism which comprises a housing, a motor attached to the housing andtransmitting an auxiliary steering force to a rotary shaft through amotor shaft, a bearing provided in the housing and rotatably supportingthe rotary shaft by use of a shaft support hole, a worm being integralwith or fitted on the rotary shaft and having a gear portion formed of ametal or a resin, an output shaft transmitting a steering force forsteering an axle and rotatably supported in a predetermined position ofthe housing, and a worm wheel being integral with or fitted on theoutput shaft in a way that meshes with the worm and having a gearportion composed of a resin, the worm gear mechanism transmitting theauxiliary steering force of the motor to the output shaft, wherein themotor is installed with respect to the output shaft in such a positionthat a length which is the sum of a working radius of the worm and aworking radius of the worm wheel becomes an axis-to-axis distancebetween the output shaft and the motor shaft, and the bearing isinstalled with respect to the output shaft in such a position that anaxis-to-axis distance between the shaft support hole and the outputshaft becomes slightly smaller than an axis-to-axis distance between theoutput shaft and the motor shaft.

Further, it is preferable that the bearing be supported on the rotaryshaft through an elastic member, and the worm be slightly movable in anaxial direction.

The apparatus being thus constructed, the rotary shaft is installedeccentrically towards the worm wheel from the axis of the motor, andtherefore it follows that the worm shaft is pressed against the elasticmember when assembled. This pressing force produces an elastic pre-loadfor pressing the worm against the worm wheel, whereby the worm mesheswith the gear portion of the worm wheel with no backlash. Accordingly,the worm and the gear portion of the worm wheel mesh with each other bya proper frictional force, and the backlash is eliminated without anydecline of a power transmitting performance.

Furthermore, according to a second invention of the present invention,an electric power steering apparatus with a worm gear mechanism whichcomprises a housing, a motor attached to the housing and transmitting anauxiliary steering force to a rotary shaft through a motor shaft, a wormformed on or fitted on the rotary shaft and having a gear portioncomposed of a metal or a resin, bearings provided in the housing,disposed respectively in positions on both sides of the worm androtatably supporting the rotary shaft, an output shaft transmitting asteering force for steering an axle and rotatably supported in apredetermined position of the housing, and a worm wheel formed on orfitted on the output shaft in a way that meshes with the worm and havinga gear portion formed of a resin, the motor installed in such a positionthat a length which is the sum of a working radius of the worm and aworking radius of the worm wheel becomes an axis-to-axis distancebetween the output shaft and the motor shaft, wherein elastic membersare disposed on both sides, in the axial direction, of the motor-sidedbearing adjacently thereto so that the rotary shaft may be slightlymovable in the axial direction within a limit of elasticity of theelastic members, the bearing positioned away from the motor is a rollingbearing, an elastic portion for biasing the worm in the meshingdirection is provided on the rotary shaft, an outer ring of the bearingis fixedly fitted in a cylindrical bearing holding member fixedly fittedin the housing, its inner ring loosely receives therein the rotatingshaft on which a cylindrical buffer member is fixedly fitted, and theelastic portion is constructed of a biasing member axially rotatablysupporting the rotary shaft, and of an elastic body receives therein,the biasing member in a position eccentric in the meshing direction ofthe worm with respect to an axis of the bearing and fixed in thevicinity of the bearing holding member.

The apparatus being thus constructed, the rotary shaft is set in theposition eccentric in the meshing direction of the worm by use of theelastic portion. Therefore, the rotary shaft is pressed against thebuffer member of the bearing when the worm is assembled, and an elasticpre-load for pressing the worm against the worm wheel is produced.Accordingly, the worm and the gear portion of the worm wheel mesh witheach other by the proper frictional force, and the backlash iseliminated without any decline of the power transmitting performance.

Particularly, a rate at which a volume of the elastic body occupies theelastic portion is taken large, whereby an initial eccentric quantityneeded for generating the pre-load can be taken large and a springconstant of the elastic body can be decreased. Therefore, even when aconfiguration of the worm might change due to a scatter in workingaccuracy and an abrasion of the gear, it is feasible to stably maintaina fixed pre-load force and to effectively prevent the tooth-buttingnoises of the gears.

Further, the rotary shaft is movable in the axial directions, and hence,when a force is applied to the rotary shaft, the rotary shaft moves inthe axial directions within the limit of the elasticity of the elasticmember, whereby the worm and the gear portion of the worm wheel meshwith each other in the proper positions to absorb an impact, and thetooth-butting noises are thus reduced.

Moreover, the bearing receives a load and a rotational torque acting inthe meshing direction of the worm, which are generated when driving(assisting) the apparatus, and thus controls a displacement of the worm.Therefore, neither a large distortion nor load occurs in the elasticbody of the elastic portion, which leads to an improvement of a lifetimeof the elastic body.

Still further, just when the rotary shaft abuts on the shaft supporthole of the bearing, a displacement in a meshing opposite direction cannot be made, and therefore a gear meshing area does not excessivelydecrease, thereby making it possible to prevent a decline of strength ofthe gear.

Furthermore, according to a third invention of the present invention, anelectric power steering apparatus comprises a housing, a motor attachedto the housing and transmitting an auxiliary steering force to a rotaryshaft through a motor shaft, a worm formed on or fitted on the rotaryshaft and having a gear portion formed of a metal or a resin, rollingbearings provided in the housing, disposed respectively in positions onboth sides of the worm and rotatably supporting the rotary shaft, anoutput shaft transmitting a steering force for steering an axle androtatably supported in a predetermined position of the housing, and aworm wheel formed on or fitted on the output shaft in a way that mesheswith the worm and having a gear portion formed of the resin, wherein apre-load applying mechanism for applying a pre-load acting towards theworm wheel is provided at the shaft side end portion, distal from themotor, of the rotary shaft.

According to the construction of the third invention, the pre-loadapplying mechanism sets the rotary shaft in the position eccentric inthe meshing direction of the worm, so that the worm and the gear portionof the worm wheel mesh with each other by the proper frictional force,and the backlash is eliminated without any decline of the powertransmitting performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an electric power steering apparatus,showing a first embodiment of a first invention of the presentinvention;

FIG. 2A is a partial enlarged view of a portion A in FIG. 1; FIG. 2B isa sectional view of a portion of an elastic member in FIG. 1; FIG. 2Cshows a section taken along the line C-C in FIG. 2B;

FIG. 3 is a sectional view of an electric power steering apparatus,showing a second embodiment of the first invention of the presentinvention;

FIG. 4 is a partial enlarged view showing a bearing portion in FIG. 3;

FIG. 5 is a sectional view of an electric power steering apparatus,showing a third embodiment of the first invention of the presentinvention;

FIG. 6 is a sectional view of an electric power steering apparatus,showing a fourth embodiment of the first invention of the presentinvention;

FIG. 7 is a sectional view showing a modified example of a splineconnecting portion in the electric power steering apparatus in FIG. 6;

FIG. 8 is a sectional view showing a first modified example of theelastic member in the electric power steering apparatus in FIG. 6;

FIG. 9 is a sectional view showing a second modified example of theelastic member in the electric power steering apparatus in FIG. 6;

FIG. 10A is a sectional view showing a third modified example of theelastic member in the electric power steering apparatus in FIG. 6; FIG.10B is a sectional view taken along the line A-A in FIG. 10A;

FIG. 11 is a sectional view of an electric power steering apparatus,showing a fifth embodiment of the first invention of the presentinvention;

FIG. 12 is a partial side view of a male spline portion of a motorshaft;

FIG. 13A is a sectional view of an electric power steering apparatus,showing a first embodiment of a second invention of the presentinvention; FIG. 13B is a sectional view taken along the line A-A in FIG.13A;

FIG. 14A is a partial sectional view of an electric power steeringapparatus, showing a second embodiment of the second invention of thepresent invention; FIG. 14B is an enlarged view of a principal portionin FIG. 14A;

FIG. 15A is a partial sectional view of the electric power steeringapparatus, showing a third embodiment of the second invention of thepresent invention; FIG. 15B is a sectional view taken along the line B-Bin FIG. 15A;

FIG. 16A is a partial sectional view of an electric power steeringapparatus, showing a fourth embodiment of the second invention of thepresent invention; FIG. 16B is a sectional view taken along the line C-Cin FIG. 16A;

FIG. 17A is a partial sectional view of an electric power steeringapparatus, showing a fifth embodiment of the second invention of thepresent invention; FIG. 17B is a sectional view taken along the line E-Ein FIG. 17A; and

FIG. 18A is a partial sectional view of an electric power steeringapparatus, showing a sixth embodiment of the second invention of thepresent invention; FIG. 18B is a sectional view taken along the line D-Din FIG. 18A.

THE EMBODIMENTS OF THE INVENTION

An embodiment of a first invention of the present invention willhereinafter be described with reference to the drawings.

FIG. 1 is a sectional view of a configuration of an electric powersteering apparatus, showing the first embodiment of the first inventionof the present invention. FIG. 2A is a partial enlarged view of aportion A in FIG. 1. FIG. 2B is a sectional view showing a portion of anelastic member in FIG. 1.

In an electric power steering apparatus 100 in FIG. 1, an electric motor10, ball bearings 3 a, 3 b for rotatably supporting a worm shaft 2, anoutput shaft 5 of a worm wheel 4, etc., are disposed or fixed inpredetermined positions in a housing 1.

The worm shaft 2 is constructed of a worm 2 a formed substantially on acentral portion thereof, bearing support portions 2 b formed on bothsides of this worm 2 a, a serrated portion 2 c formed at one end portion(at a right end portion in the Figure) of the worm shaft 2, and soforth. On the other hand, a serrated hole 10 b is formed inside a motorshaft 10 a of the electric motor 10. The serrated portion 2 c of theworm shaft 2 is loosely fitted in the serrated hole 10 b, whereby theworm shaft 2 is joined to the motor shaft 10 a in a way that is movablein the axial directions but unmovable in a rotational direction.

As illustrated in FIG. 1 and 2A, the bearing support portions 2 b, 2 bare fitted in inner peripheral surfaces of the ball bearings 3 a, 3 b.An annular groove 2 e is formed in an axially central portion of eachbearing support portion 2 b, and an annular elastic member 6 is fittedwith no clearance on this annular groove 2 e.

A diameter of an outer peripheral surface of the elastic member 6 is setslightly larger than an outside diameter r of the bearing supportportion 2 b. This annular elastic member 6 includes a rubber bush 6 a asits body of which an inside-diametrical portion takes a bellows-likeshape and is fitted on a bottom peripheral surface of the annular groove2 e, and a ring-like material (for example, Teflon (registeredtrademark) material) 6 b that has a small coefficient of friction and iswelded to an outer periphery of the rubber bush 6 a in order to allowthe worm shaft 2 to move in the axial directions.

The worm wheel 4 is fixedly fitted on the output shaft 5 extending in adirection orthogonal to the axial direction of the worm shaft 2 and is,the output shaft 5 being rotatably supported in a predetermined positionof the housing 1 in a state of the worm wheel 4 meshing with the worm 2a, thus disposed. A gear portion 4a of the worm wheel 4 is formed of aresin.

As illustrated in FIG. 1, in the meshing between the worm 2 a and theworm wheel 4, let S1(a+b=S1) be a distance which is the sum of a workingradius a of the worm 2 a and a working radius b of the worm wheel 4, andthe output shaft 5 and the electric motor 10 are disposed in the housing1 so that an axis-to-axis distance between the motor shaft 10 a of theelectric motor 10 and the output shaft 5 of the worm wheel 4 becomes S1.On the other hand, the bearing 3 a on a distal side from the electricmotor 10 is disposed in the housing 1 in such a position that anaxis-to-axis distance S2 between a shaft support hole 3 c of the bearing3 a and the output shaft 5 becomes slightly smaller than theaxis-to-axis distance S1 between the output shaft 5 and the motor shaft10 a. In the present embodiment, these distances S1, S2 are set such asS1=47.5 mm ad S2=47.2 mm, and a difference ΔS between the axis-to-axisdistance S1 and the axis-to-axis distance S2 is given by (S1=S2=ΔS). Anoptimal value of this difference ΔS is 0.1 mm through 0.5 mm.

In this connection, as shown in FIG. 2, the outside diameter r of eachof the bearing support portions 2 b rotatably supported on the ballbearings 3 a, 3 b of the worm shaft 2, is set to a dimension given by (ashaft support hole 3 c inside diameter R−2·ΔS). Along with this, athickness of the elastic member 6 is set to a value enough to permitloose fitting therein of the bearing support portion 2 b of the wormshaft 2.

In this construction, the axis of the shaft support hole 3 c of each ofthe ball bearings 3 a, 3 b is set, as described above, eccentric by ΔStowards the worm wheel 4. Therefore, when incorporated, the worm 2 a andthe gear portion 4 a of the worm wheel 4 mesh with each other withoutany backlash, however, with reaction thereof, it follows that the wormshaft 2 (the bearing support portion 2 b) is pressed against the annularelastic member 6. This pressing force produces an elastic pre-load forpressing the worm 2 a against the worm wheel 4, while the worm 2 a iskept in a so-called floating state.

This pre-load force generates a friction to some extent when the worm 2a meshes with the worm wheel 4, however, the thickness and a rigidity ofthe annular elastic member 6 are set so that a resistance force thereofdoes not become excessively large enough to cause a hindrance to a gearperformance or to such a degree that a deviation in meshing does notoccur due to an input of vibrations applied from tires. The rigidity ofthe elastic member 6 can be set depending on hardness and aconfiguration of the rubber without any restriction.

Further, the worm shaft 2 is movable in the axial directions, and hencethe worm 2 a and the gear portion 4 a of the worm wheel 4 can be meshedwith each other in their proper positions.

Thus, the inside diameter of the shaft support hole 3 c of each of theball bearings 3 a, 3 b is set eccentric, the diameter of the worm shaft2 (the bearing support portion 2 b) is set comparatively small, and theannular elastic members 6 b, are provided between the worm shaft 2 andthe bearings 3 a, 3 b, which all cooperate to enable a pre-loadmechanism to be easily structured and the backlash to be eliminated.

According to the first embodiment of the first invention, both ofeccentric quantities of the inside diameters of the shaft support holes3 c of the bearings 3 a, 3 b are set likewise to ΔS, however, theeccentric quantity of the bearing 3 b on the proximal side to the motor10 can be set smaller than the eccentric quantity ΔS of the bearing 3 b.Namely, the distance between the axis of the bearing 3 a on the distalside from the motor and the axis of the output shaft 5, can be setlarger than the distance between the axis of the bearing 3 b on theproximal side to the motor and the axis of the output shaft 5. It isdesirable that a difference ΔS′ between the eccentric quantities be avalue of 0 through ΔS/2. This construction is more desirable in terms ofpreventing the axis deviation between the motor shaft 10 a and the wormshaft 2 by reducing the rotational resistance of the worm shaft 2.

Next, a second embodiment of the first invention of the presentinvention will be described in conjunction with the drawings.

FIG. 3 is a sectional view of a construction of the electric powersteering apparatus, showing the second embodiment of the first inventionof the present invention. FIG. 4 is an enlarged view showing a bearingportion in FIG. 3.

Throughout the following discussion, the components having the similarstructures as those in the first embodiment of the first invention willbe explained by use of the same numerals or symbols.

In the electric power steering apparatus 100 in FIG. 3, the electricmotor 10, the ball bearings 3 a, 3 b for rotatably supporting the wormshaft 2, the output shaft 5 of the worm wheel 4, etc., are disposed orfixed in predetermined positions in the housing 1.

The worm shaft 2 is constructed of the worm 2 a formed substantially onthe central portion thereof, the bearing support portions 2 b formed onboth sides of this worm 2 a, the serrated portion 2 c formed at one endportion (at a right end portion in the Figure) of the worm shaft 2, andso forth. On the other hand, the serrated hole 10 b is formed inside themotor shaft 10 a of the electric motor 10. The serrated portion 2 c ofthe worm shaft 2 is loosely fitted in the serrated hole 10 b, wherebythe worm shaft 2 is joined to the motor shaft 10 a in a way that ismovable in the axial directions but unmovable in the rotationaldirection.

As shown also in FIG. 4, annular elastic members 60 each having apredetermined thickness and predetermined elasticity are respectivelyfitted with no gap in the inner peripheral surfaces of the shaft supportholes 3 c of the ball bearings 3 a, 3 b. The annular elastic member 60is constructed of a collar portion 60 a and a cylindrical portion 60 b,wherein the cylindrical portion 60 b is fitted in the shaft support hole3 c, and the bearing support portion 2 b of the worm shaft 2 is looselyfitted in the cylindrical portion 60 b. Accordingly, the worm shaft 2 isrotatably supported by the ball bearings 3 a, 3 b through the elasticmembers 60 in these bearing support portions 2 b.

Elastic member support members 7 are fixedly fitted on the worm shaft 2,wherein these members 7 are disposed adjacent to the pair of bearingsupport portions 2 b respectively on the side of the worm 2 a. Theseelastic member support members 7 have a function of holding the collarportions 60 a of the elastic members 60, respectively between thebearings 3 a, 3 b and the support members 7 and allowing displacements,in the axial directions (in the right-and-left directions in theFigure), of the worm shaft 2 within a limit of the elasticity of theelastic members 60.

The worm wheel 4 is fixedly fitted onto the output shaft 5 extending inthe direction orthogonal to the axial direction of the worm shaft 2 andis, the output shaft 5 being rotatably supported in the predeterminedposition of the housing 1 in the state of the worm wheel 4 meshing withthe worm 2 a, thus disposed. The gear portion 4 a of the worm wheel 4 isformed of a resin.

As illustrated in FIG. 3, in the meshing between the worm 2 a and theworm wheel 4, let S1(a+b=S1) be a distance which is the sum of theworking radius a of the worm 2 a and the working radius b of the wormwheel 4, and the output shaft 5 and the electric motor 10 are disposedin the housing 1 so that the axis-to-axis distance between the motorshaft 10 a of the electric motor 10 and the output shaft 5 of the wormwheel 4 becomes S1. On the other hand, the bearings 3 a, 3 b aredisposed in the housing 1 in such positions that the axis-to-axisdistance S2 between the shaft support hole 3 c thereof and the outputshaft 5 becomes slightly smaller than the axis-to-axis distance S1between the output shaft 5 and the motor shaft 10 a. The difference ΔSbetween the axis-to-axis distance S1 and the axis-to-axis distance S2 isgiven by (S1−S2=ΔS). This difference As is set to an optimal value of0.1 mm through 0.5 mm.

In this connection, as shown in FIG. 4, the outside diameter r of eachof the bearing support portions 2 b rotatably supported on the ballbearings 3 a, 3 b of the worm shaft 2, is set to a dimension given by (ashaft support hole 3 c inside diameter R−2·ΔS) . Along with this, athickness of the elastic member 60 is set to a value to the extent thatthe bearing support portion 2 b of the worm shaft 2 is loosely fittedtherein.

In this construction, the axis of the shaft support hole 3 c of each ofthe ball bearings 3 a, 3 b is set, as described above, eccentric by ΔStowards the worm wheel 4. Hence, when incorporated, the worm 2 a and thegear portion 4 a of the worm wheel 4 mesh with each other without anybacklash, however, with the reaction thereof, it follows that the wormshaft 2 (the bearing support portion 2 b) is pressed against acylindrical portion 60 b of the annular elastic member 60. This pressingforce produces an elastic pre-load for pressing the worm 2 a against theworm wheel 4, and therefore the worm 2 a is kept in the so-calledfloating state.

This pre-load force generates the friction to some extent when the worm2 a meshes with the worm wheel 4, however, the thickness and a rigidityof the elastic member 60 are set so that the resistance force thereofdoes not become excessively large enough to cause the hindrance to thegear performance or to such a degree that the deviation in meshing doesnot occur due to the input of vibrations applied from tires. Therigidity of the elastic member 60 can be set depending on the hardnessand the configuration of the rubber without any restriction.

Further, the worm shaft 2 is movable in the axial directions within thelimit of the elasticity of the collar portion 60 a of the elastic member60, and hence the worm 2 a and the gear portion 4 a of the worm wheel 4can be meshed with each other in their proper positions.

Thus, the inside diameter of the shaft support hole 3 c of each of theball bearings 3 a, 3 b is set eccentric, the diameter of the worm shaft2 (the bearing support portion 2 b) is set comparatively small, and theelastic member 60 is provided between the worm shaft 2 and the bearings3 a, 3 b, which all cooperate to enable a pre-load mechanism to beeasily structured and the backlash to be eliminated.

Next, a third embodiment of the first invention of the present inventionwill be described.

FIG. 5 is a sectional view of the electric power steering apparatus,showing the third embodiment of the first invention of the presentinvention. The third embodiment of the first invention has the sameconstruction as the second embodiment of the first invention has, exceptfor points that will hereinafter be explained. Therefore, the similarcomponents as those in the second embodiment of the first invention areillustrated in a way that marks them with the same numerals or symbols,and their repetitive explanations are omitted.

In the second embodiment of the first invention discussed above, both ofthe eccentric quantities of the inside diameters of the shaft supportholes 3 c of the bearings 3 a, 3 b are set likewise to ΔS, however,according to the third embodiment of the first invention, the eccentricquantity of the bearing 3 b on the proximal side to the motor 10 can beset smaller than the eccentric quantity ΔS of the bearing 3 b. Namely,in the third embodiment of the first invention illustrated in FIG. 5, adistance S3 between the axis of the bearing 3 a on the distal side fromthe motor and the axis of the output shaft 5, is set larger than adistance S4 between the axis of the bearing 3 b on the proximal side tothe motor and the axis of the output shaft 5. It is desirable that adifference ΔS′ (S3−S4=ΔS′) between S3 and S4 be a value of 0 throughΔS/2. This construction is more desirable in terms of preventing theaxis deviation between the motor shaft 10 a and the worm shaft 2 byreducing the rotational resistance of the worm shaft 2.

Next, a fourth embodiment of the first invention of the presentinvention will be discussed with reference to the drawings.

FIG. 6 is a sectional view of the electric power steering apparatus,showing the fourth embodiment of the first invention of the presentinvention. FIG. 7 is a sectional view showing a modified example of aspline connecting portion in the electric power steering apparatus inFIG. 6. FIG. 8 is a sectional view showing a first modified example ofthe elastic member in the electric power steering apparatus in FIG. 6.FIG. 9 is a sectional view showing a second modified example of theelastic member in the electric power steering apparatus in FIG. 6. FIG.10A is a sectional view showing a third modified example of the elasticmember in the electric power steering apparatus in FIG. 6. FIG. 10B is asectional view taken along the line A-A in FIG. 10A.

In the electric power steering apparatus 100 in FIG. 6, the electricmotor 10, the ball bearings 3 a, 3 b for rotatably supporting the wormshaft 2 defined as a rotary shaft, the output shaft 5 of the worm wheel4, etc., are disposed or fixed in predetermined positions in the housing1.

The worm shaft 2 is constructed of the worm 2 a formed substantially onthe central portion thereof, and the bearing support portions 2 b formedon both sides of this worm 2 a. A connecting member 18 taking acylindrical shape is fixedly fitted on the bearing support portion 2 b(on the right side in the Figure) on the side of the bearing 3 bproximal to the electric motor 10. A female spline portion 18 a isformed substantially in a half, in the axial direction, of an innerperipheral surface of the connecting member 18. The connecting member 18is fitted in the shaft support hole 3 c (an inner peripheral surface ofan inner ring) of the bearing 3 b so that this connecting member 18 ismovable in the axial directions.

On the other hand, a male spline portion 10 b is provided on a front endportion of the motor shaft 10 a of the electric motor 10. This malespline portion 10 b is loosely fitted in the female spline portion 18 aof the connecting member 18, whereby the worm shaft 2 isspline-connected to the motor shaft 10 a in a state of being movable inthe axial directions but unmovable in the rotational direction. Thisspline connecting portion is set so that substantially a half of thisconnecting portion is positioned within the shaft support hole 3 c ofthe bearing 3 b. The spline connecting portion is set so that at leastthe half thereof in the axial direction, in other words, the half ormore of the spline connecting portion in the axial direction ispositioned within the shaft support hole 3 c of the bearing 3 b.

A part of an elastic member 16 taking substantially the cylindricalshape is fixedly fitted in a shaft support hole 3 d of the bearing 3 aon the distal side from the electric motor 10. This elastic member 16 isconstructed of a fitting portion 16 a, a large diametrical portion 16 b,of which an outside diameter is larger than a diameter of the shaftsupport hole 3 d, formed adjacent to the fitting portion 16 a, and ashaft support portion 16 c formed in continuation from the largediametrical portion 16 a and having an inside diameter substantiallyequal to an outside diameter of the bearing support portion 2 b. Afastening ring 11 for fixedly fitting the shaft support portion 16 c onthe bearing support portion 2 b of the worm shaft 2, is fitted on anouter peripheral portion of the shaft support portion 16 c.

This axial support portion 16 c has a function of elastically holdingthe bearing support portion 2 b of the worm shaft 2 substantially in anintermediate position within the inside diameter of the fitting portion16 a. The large diametrical portion 16 b has a function of facilitatingthe axial movements of the worm shaft 2 by a contrivance that the shaftsupport portion 16 c extends and shrinks in the axial directions whenmoving together with the bearing support portion 2 b in the axialdirections.

An inside diameter R1 (see also FIG. 8) of the fitting portion 16 a ofthe elastic member 16 is set slightly larger than the outside diameter rof the bearing support portion 2 b rotatably supported by the bearing 3a. The outside diameter r of the bearing support portion 2 b is set to adimension given by (a fitting portion 16 a inside diameter R1−2·ΔS).This is a value of such a degree that the bearing support portion 2 b ofthe worm shaft 2 is loosely fitted in the fitting portion 16 a.

On the other hand, elastic members 17 each assuming substantially aring-shape are disposed on both sides of the bearing 3 b proximal to theelectric motor 10 in the axial direction adjacently to the bearing 3 b.These two elastic members 17 are so disposed that each is held by twopieces of ring-shaped holding members 17 a and 17 b (see also FIG. 7).The holding members 17 a positioned away from the bearing 3 b are eachfixedly fitted on the connecting member 18. The holding members 17 bpositioned on the side of the bearing 3 b are fixed to the bearing 3 band provided along the connecting member 18 a in a non-contact manner.The elastic member 17 elastically extends and shrinks in the axialdirections as the connecting member 18 moves in the axial directions,thereby allowing the worm shaft 2 to move in the axial directions withinthe limit of the elasticity thereof.

The worm wheel 4 is fixedly fitted on the output shaft 5 extending inthe direction orthogonal to the axial direction of the worm shaft 2. Theoutput shaft 5 is so disposed as to be rotatably supported in thepredetermined position of the housing 1 in the state of the worm wheel 4meshing with the worm 2 a. The gear portion 4 a of the worm wheel 4 isformed of a resin.

As illustrated in FIG. 6, in the meshing between the worm 2 a and theworm wheel 4, let S1 (a+b=S1) be a distance which is the sum of theworking radius a of the worm 2 a and the working radius b of the wormwheel 4, and the output shaft 5 and the electric motor 10 are disposedin the housing 1 so that the axis-to-axis distance between the motorshaft 10 a of the electric motor 10 and the output shaft 5 of the wormwheel 4 becomes S1. On the other hand, the bearing 3 a on the distalside from the electric motor 10 is disposed in the housing 1 in such aposition that the axis-to-axis distance S2 between the shaft supporthole 3 d of the bearing 3 a and the output shaft 5 becomes slightlysmaller than the axis-to-axis distance S1 between the output shaft 5 andthe motor shaft 10 a. In this embodiment, the distances S1, S2 are setsuch as S1=47.5 mm, and S2=47.2 mm. It is effective that the differenceΔS(S1−S2=ΔS) between the axis-to-axis distance S1 and the axis-to-axisdistance S2 be set to an optimal value in a range of 0.1 mm through 0.5mm.

In the construction described above, the axis of the shaft support hole3 d of the bearing 3 a is set, as described above, eccentric by ΔStowards the worm wheel 4. Hence, when incorporated, the worm 2 a and thegear portion 4 a of the worm wheel 4 mesh with each other without anybacklash, however, with the reaction thereof, it follows that the wormshaft 2 (the bearing support portion 2 b) is pressed against the elasticmember 6. This pressing force produces an elastic pre-load for pressingthe worm 2 a against the worm wheel 4. Thus, the worm 2 a is kept in theso-called floating state.

This pre-load force generates the friction to some extent when the worm2 a meshes with the worm wheel 4, however, the thickness and therigidity of the elastic member 16 are set so that the resistance forcethereof does not become excessively large enough to cause the hindranceto the gear performance or to such a degree that the deviation inmeshing does not occur due to the input of vibrations applied fromtires. The rigidity of this elastic member 16 can be set depending onthe hardness and the configuration of the rubber without anyrestriction.

Further, the worm shaft 2 is movable in the axial directions, and hence,when a force is applied from the tire, the worm shaft 2 moves in theaxial directions within the limit of the elasticity of the elasticmember 7, whereby the worm 2 a and the gear portion 4 a of the wormwheel 4 mesh with each other in proper positions to make it possible toabsorb an impact.

Incidentally, the worm shaft 2 is movable in directions of meshingbetween the worm 2 a and the worm wheel 4 at a portion of the bearing 3a on the distal side from the electric motor 10. Therefore, the wormshaft 2 has a possibility of causing a hindrance to the axial movementsof the worm shaft 2 due to an axis deviation occurred in the splineconnecting portion, wherein the bearing 3 b on the proximal side to theelectric motor 10 serves as a fulcrum.

According to the fourth embodiment of the first invention, however,substantially the half, in the axial direction, of the spline connectingportion between the worm shaft 2 and the motor shaft 10 a is so set asto be positioned within the shaft support hole 3 c of the bearing 3 b.Hence, even when the worm shaft 2 slightly moves in the meshingdirection of the worm 2 a, with the bearing 3 b serving as the fulcrum,a displacement of the worm shaft 2 is allowed owing to a slight play ofthe spline connecting portion, and the worm shaft 2 can move in theaxial directions without wresting the spline connecting portion.

Thus, the inside diameter of the shaft support hole 3 c of the ballbearing 3 a is set eccentric, the diameter of the worm shaft 2 (thebearing support portion 2 b) is set comparatively small, and the elasticmembers 16, 17 are provided between the worm shaft 2 and the bearings 3a, 3 b, which all cooperate to enable a pre-load mechanism to be easilystructured, the backlash to be completely eliminated, the impact to beabsorbed and the tooth-butting noises (rattle noises) to be restrained.

Note that the fourth embodiment of the first invention has involvesusing the connecting member 18 for the spline connecting portion. Theinvention is not, however, limited to this construction and may takesuch a construction that, as shown in FIG. 7, a bearing support portion2 b′ on the side of the bearing 3 b is molded to have substantially thesame diameter as the outside diameter of the connecting member 18, aconnecting hole 2 f including a female spline portion 2 e for the motorshaft 10 a, and a male spline portion 10 d of the motor shaft 10 a isinserted into this connecting hole 2 f, thus attaining a splineconnection. In this case also, the holding members 17 a, 17 b areattached to the bearing support member 2 b′ with the same configurationas in the embodiment of the connecting member 18 described above.Referring to FIG. 7, the same members as those of the apparatus in FIG.6 are marked with the same numerals or symbols.

Next, modified examples of the elastic member 16 will be describedreferring to FIGS. 8 to 10B inclusive.

FIG. 8 shows a first modified example of the elastic member 16.Referring to FIG. 8, the same members as those in the embodiment in FIG.6 are marked with the same numerals or symbols, and the repetitiveexplanations thereof are omitted. A cylindrical elastic member 160 isconstructed of a fitting portion 160 a (which is the same as that of theelastic member 16) serving as a portion that loosely fits therein thebearing support portion 2 b and is fixedly fitted in the shaft supporthole 3 c of the bearing 3 a, a shaft support portion 160 b adjacent tothe fitting portion 160 a, having substantially the same inside diameteras the outside diameter of the bearing support portion 2 b and receivingtherein the bearing support portion 2 b with no gap therebetween, and atruncated conic portion 160 c for integrally connecting the fittingportion 160 a and the shaft support portion 160 b. Namely, the elasticmember 160 has a configuration that an axial length of the shaft supportportion 16 c is extended instead of a configuration, the largediametrical portion 16 b is removed from the elastic member 16 describedabove, and no fastening ring 11 is provided.

A relationship between the inside diameter R1 of the fitting portion 160a of the cylindrical elastic member 160 and the outside diameter r ofthe bearing support portion 2 b is the same as in the embodiment in FIG.6, and hence its repetitive explanation is omitted.

In this construction, the shaft support portion 160 b has a function ofelastically holding the bearing support portion 2 b substantially in anintermediate position within the inside diameter of the fitting portion160 a. It is required that an inside diameter of the axial supportportion 160 b be set to such a dimension as to make an inner ring of thebearing 3 a rotatable as the shaft support portion 160 b rotates withrotations of the bearing support portion 2 b and as to make the bearingsupport portion 2 b smoothly slidable when the bearing support portion 2b moves in the axial directions. It can be expected that this elasticmember 160 exhibits the same effects as the elastic member 16 does.

FIG. 9 shows a second modified example of the elastic member 16.Referring to FIG. 9, the same members as those in the embodiment in FIG.6 are marked with the same numerals or symbols, and the repetitiveexplanations thereof are omitted. A cylindrical elastic member 161 isconstructed of an shaft support portion 161 a having substantially thesame inside diameter as the outside diameter of the bearing supportportion 2 b and fitting therein the bearing support portion 2 b with nogap, and a protruded portion 161 b formed as a large diametrical portionhaving a small wall thickness formed integrally with an intermediateportion, in the axial direction, of the shaft support portion 161 a, theprotruded portion 161 b being fixedly fitted in the shaft support hole 3c of the bearing 3 a. It is required that an inside diameter of theshaft support portion 161 a be set to a dimension that makes the bearingsupport portion 2 b smoothly slidable when the bearing support portion 2b moves in the axial directions. A ring-shaped stopper 19 is fixedlyfitted on a left end portion, as viewed in the Figure, of the bearingsupport portion 2 b.

In this construction, the shaft support portion 161 a permits thebearing support portion 2 b to move in the axial direction (away ortowards from the motor 10) but the stopper 19 restricts. When thebearing support portion 2 b moves in the meshing direction of the worm 2a, the protruded portion 161 b is compressed in a direction that pressesthe inner peripheral surface of the shaft support hole 3 c, therebyelastically allowing the movement.

Next, a third modified example of the elastic member 16 is shown in FIG.10A. Referring to FIG. 10A, the same members as those in the embodimentin FIG. 6 are marked with the same numerals or symbols, and theirrepetitive explanations are omitted. A cylindrical elastic member 162 isfixedly fitted in the holding member 162 a, and the holding member 162 ais fixedly fitted in the shaft support hole 3 d of the bearing 3 a. Aninside diameter of the elastic member 162 is substantially equal to theoutside diameter of the bearing support portion 2 b, and is set to adimension that makes the bearing support portion 2 b smoothly slidablewhen the bearing support member 2 b moves in the axial directions.

A multiplicity of grooves 162 b are, as illustrated in FIG. 10B, formedalong an axial direction on an inner peripheral surface of the elasticmember 162, whereby the elastic member 162 is easy to be compressed frominside its inner peripheral surface. When the bearing support portion 2b moves in the meshing direction of the worm 2 a, the bearing supportportion 2 b presses the inner peripheral surface (formed with themultiplicity of grooves 162 b) of the elastic member 162 and thuscompresses the elastic member 162 in this direction, thereby elasticallyallowing the movement.

The ring-shaped stopper 19 is fixedly fitted on the left end portion, asviewed in the Figure, of the bearing support portion 2 b. The elasticmember 162 permits the bearing support portion 2 b to move in the axialdirection (away from or towards the motor 10) but the stopper 19restricts the movement in the axial direction.

As discussed above, according to the fourth embodiment of the firstinvention of the present invention, the motor-sided bearing has theelastic members disposed on both sides in the axial direction adjacentlythereto, whereby the worm is slightly movable in the axial direction.Besides, the bearing on the distal side from the motor supports therotary shaft (the worm shaft) through the elastic member, and thisrotary shaft is slightly movable in the meshing direction of the worm.Therefore, with the simple construction, the backlash can be completelyeliminated. Further, when the force is applied to the rotary shaft, therotary shaft moves in the axial directions within the limit of theelasticity of the elastic member, with the result that the worm and thegear portion of the worm wheel mesh with each other in the properpositions to absorb the impact. It is therefore possible to reduce thetooth-butting noises without any decline of a transmission performanceof an auxiliary steering force.

Moreover, in the fourth embodiment of the first invention, at least thehalf, in the axial direction, of the spline connecting portion betweenthe rotary shaft and the motor shaft is positioned within the shaftsupport hole of the bearing. Therefore, even when the rotary shaftslightly moves in the meshing direction of the worm, as the motor-sidedbearing serves as the fulcrum, the rotary shaft can move in the axialdirection without wresting the spline connecting portion.

Furthermore, the pre-load force of the worm can be adjusted depending onthe material and the configuration of the elastic member, and hencedimensional accuracies are not excessively required of the rotary shaftand the bearing as well, thereby facilitating the dimensional control.

Next, a fifth embodiment of the first invention of the present inventionwill be explained in conjunction with FIGS. 11 and 12. FIG. 11 is asectional view of the electric power steering apparatus, showing thefifth embodiment of the first invention. FIG. 12 is a partial side viewof the male spline portion of the motor shaft.

In the fifth embodiment of the first invention, the components havingthe same structures as those in the aforementioned embodiments of thefirst invention are shown with the same numerals or symbols.

In the electric power steering apparatus 100 in FIG. 11, the electricmotor 10, the ball bearings 3 a, 3 b for rotatably supporting the wormshaft 2 defined as the rotary shaft, the output shaft 5 of the wormwheel 4, etc., are disposed or fixed in predetermined positions in thehousing 1.

The worm shaft 2 is constructed of the worm 2 a formed on the centralportion thereof, and the bearing support portions 2 b, 2 b′ formed onboth sides of this worm 2 a. A relationship between the bearing portion2 b′ proximal to the electric motor 10 and the motor shaft 10 a of theelectric motor 10 is similar to what is shown in FIG. 7. Namely, thebearing portion 2 b′ is formed with the connecting hole 2 f openedtoward the motor side and having the female spline portion 2 e. An endof the motor shaft 10 a is formed with a male spline portion 10 d′. Thefemale spline portion 2 e is spline-connected to the male spline portion10 d′.

In the fifth embodiment of the first invention, the male spline portion10 d′ of the motor shaft 10 a takes, as illustrated in FIG. 12, adrum-like shape whose diameter is small at both of its ends with anarrow key width but is large at its central portion in the axialdirection. The male spline portion 10 d′ is fitted into the bearingportion 2 b′ of the worm shaft at a central portion of the ball bearing3 b. A diameter of the female spline portion of the bearing portion 2 b′remains unchanged in the axial direction.

In the fifth embodiment of the first invention, a structure between theball bearing 3 a on the distal side from the motor 10, the bearingsupport portion 2 e of the worm shaft 2 and the cylindrical elasticmember 160 interposed therebetween, is the same as what is shown in FIG.8.

In the fifth embodiment of the first invention, structures other thanthe above-mentioned are the same as those in the embodiments discussedabove, and hence their repetitive explanations are omitted.

In the fifth embodiment of the first invention, when the electric motor10 generates an assist torque, the worm receives the force in such adirection as to get away from the wheel. At this time, the worm istilted about the central portion, serving as a fulcrum, of the bearingon the proximal side to the motor, however, the spline portions do notinterfere with each other, and neither the operating force nor a feelingat steering neutral time is deteriorated. Further, in the case where theworm moves in the axial direction, the interference between the splineportions does not occur.

Moreover, the end portion of the male spline portion 10 d′ of the motorshaft 10 a is small of its diameter, and hence when the apparatus isassembled, the male spline portion 10 d′ is easy to fit into the malespline portion 2 e, thereby improving the operability.

Next, embodiments of a second invention of the present invention will bediscussed with reference to the drawings.

FIG. 13A is a sectional view of the electric power steering apparatus,showing a first embodiment of the second invention of the presentinvention. FIG. 13B is a sectional view taken along the line A-A in FIG.13A. FIG. 14A is a sectional view of the electric power steeringapparatus, showing a second embodiment of the second invention of thepresent invention. FIG. 14B is an enlarged view of a principal portionin FIG. 14A. FIG. 15A is a sectional view of the electric power steeringapparatus, showing a third embodiment of the second invention of thepresent invention. FIG. 15B is a sectional view taken along the line B-Bin FIG. 15A. FIG. 16A is a sectional view of the electric power steeringapparatus, showing a fourth embodiment of the second invention of thepresent invention. FIG. 16B is a sectional view taken along the line C-Cin FIG. 16A. FIG. 17A is a sectional view of the electric power steeringapparatus, showing a fifth embodiment of the second invention of thepresent invention. FIG. 17B is a sectional view taken along the line E-Ein FIG. 17A. FIG. 18A is a sectional view of the electric power steeringapparatus, showing a sixth embodiment of the second invention of thepresent invention. FIG. 18B is a sectional view taken along the line D-Din FIG. 18A.

In the electric power steering apparatus 100 in FIGS. 13A and 13Bshowing the first embodiment of the second invention of the presentinvention, an electric motor 101, ball bearings 103 a, 103 c forrotatably supporting a worm shaft 102 defined as a rotary shaft, anelastic portion 103 b, an output shaft 105 of a worm wheel 104, etc.,are disposed or fixed in predetermined positions in a housing 101.

The worm shaft 102 is constructed of a worm 102 a formed substantiallyon the central portion thereof, and bearing support portions 102 bformed on both sides of the worm 102 a. A cylindrical connecting member108 is fixedly fitted on the bearing support portion 102 b (the rightside in the Figure) on the side of the ball bearing 103 c proximal tothe electric motor 110. A female spline portion 108 a is formedsubstantially in a half, in the axial direction, of an inner peripheralsurface of the connecting member 108. The connecting member 108 isfitted in a shaft support hole 103 d (an inner peripheral surface of aninner ring) of the ball bearing 103 c so that the connecting member 108is movable in the axial directions.

On the other hand, a male spline portion 110 b is provided on an endportion of the motor shaft 110 a of the electric motor 110. This malespline portion 110 b is loosely fitted in the female spline portion 108a of the connecting member 108, whereby the worm shaft 102 isspline-connected to the motor shaft 110 a in a state of being movable inthe axial directions but unmovable in the rotational direction. Thisspline connecting portion is set so that substantially a half of theconnecting portion in the axial direction is positioned within the shaftsupport hole 103 d of the bearing 103 c. The connecting member 108 maybe formed integrally with the worm 102 a.

On the opposite side to the motor, the ball bearing 103 a and an elasticportion 103 b are disposed side by side in the axial direction. Thebearing 103 a has a function as a rolling bearing, and an outer ringthereof is fixedly fitted in a bearing ring 114 defined as a cylindricalbearing holding member fixedly fitted in the housing 101. A cylindricalbuffer member 106 formed of a resin, etc., which is fixedly fitted onthe bearing support portion 102 b of the worm shaft 102, is looselyfitted in an inner ring of the bearing 103 a.

The elastic portion 103 b is constructed of a biasing member 112 havingan inside diameter substantially equal to an outside diameter of thebearing support portion 102 b and being formed of resin, and an elasticbody 113 fixedly fitting therein the biasing member 112 and fittingrein, fixedly fitted in the bearing ring 114 and formed of a rubber,etc. A position in which the elastic body 113 receives therein thebiasing member 112 is far eccentric in the meshing direction of the worm102 a. When incorporating the worm shaft 102 into the ball bearing 103 aand the elastic portion 103 b, however, the worm shaft 102 becomessubstantially concentric with the bearing 103 a, and hence the elasticbody 113 deforms in a direction opposite to the meshing direction of theworm 102 a, thereby producing a pre-load force in the meshing direction.

An outside diameter of the buffer member 106 is set slightly smallerthan an inside diameter of the inner ring of the bearing 103 a. Let ΔSbe a displacement quantity of the worm 102 a in the meshing directionfor eliminating the backlash, and an outside diameter of the buffermember 106 is set to a dimension given by (an inner ring inside diameter−2·ΔS). This is a value of such a degree that the buffer member 106 isloosely fitted into the inner ring.

On the other hand, elastic members 107 each assuming substantially aring-shape are disposed on both sides of the bearing 103 c proximal tothe electric motor 110 in the axial direction adjacently to the bearing103 c. These two elastic members 107, 107 are disposed in the form ofeach being held by two pieces of ring-shaped holding members 107 a and107 b. The holding members 107 a positioned away from the bearing 103 care each fixedly fitted on the connecting member 108. The holdingmembers 107 b abutting on the bearing 103 c are fixed to the bearing 103b and provided along the connecting member 108 in a non-contact manner.The elastic member 107 elastically extends and shrinks in the axialdirections as the connecting member 108 moves in the axial directions,thereby allowing the worm shaft 102 to move in the axial directionswithin the limit of the elasticity thereof.

The worm wheel 104 is fixedly fitted on the output shaft 105 extendingin the direction orthogonal to the axial direction of the worm shaft102. The output shaft 105 is so disposed as to be rotatably supported ina predetermined position of the housing 101 in a state of the worm wheel104 meshing with the worm 102 a. A gear portion 104 a of the worm wheel104 is formed of a resin.

As illustrated in FIG. 13A, in the meshing between the worm 102 a andthe worm wheel 104, let S (a+b=S) be a distance which is the sum of aworking radius a of the worm 102 a and a working radius b of the wormwheel 104, and the output shaft 105, the bearing 103 c and the electricmotor 110 are disposed in the housing 101 so that an axis-to-axisdistance between the motor shaft 110 a of the electric motor 110 and theoutput shaft 105 of the worm wheel 104 and an axis-to-axis distancebetween the bearing 103 c and the output shaft 105 become S. In thisembodiment, the distance S is set such as S=47.5 mm. It is effectivethat the displacement quantity ΔS of the worm 102 a in the meshingdirection for eliminating the backlash be set to an optimal value in arange of 0.1 mm through 0.5 mm.

In the construction described above, a position in which the biasingmember 112 is fitted in the elastic body 113 of the elastic portion 103b, is eccentric in the meshing direction of the worm 102 a, and hence,when assembling the worm shaft 102 into the bearing 103 a, the elasticbody 113 deforms to produce a pre-load force in the meshing direction ofthe worm 102 a. Then, the worm 102 a and the gear portion 104 a of theworm wheel 104 mesh with each other without the backlash. Thus, the worm102 a is kept in the so-called floating state.

Particularly, a rate at which a volume of the elastic body 113 occupiesthe elastic portion 103 b is taken large, whereby an initial eccentricquantity needed for generating the pre-load can be taken large and aspring constant of the elastic body 113 can be decreased. Therefore,even when a configuration of the worm 102 a might change due to ascatter in working accuracy and an abrasion of the gear, it is feasibleto stably maintain a fixed pre-load force and to effectively prevent thetooth-butting noises of the gears.

This pre-load force generates the friction to some extent when the worm102 a meshes with the worm wheel 104, however, a thickness and arigidity of the elastic body 113 are set so that a resistance forcethereof does not become excessively large enough to cause a hindrance toa gear performance or to such a degree that a deviation in meshing doesnot occur due to an input of vibrations applied from tires. The rigidityof this elastic body 113 can be set depending on hardness and aconfiguration of the rubber without any restriction.

Further, the worm shaft 102 is movable in the axial directions, andhence, when a force is applied from the tire, the worm shaft 102 movesin the axial directions within a limit of elasticity of the elasticmember 107, whereby the worm 102 a and the gear portion 104 a of theworm wheel 104 mesh with each other in proper positions to make itpossible to absorb an impact.

In the portion of the ball bearing 103 a, even in a case where apressure is exerted on the buffer member 106 due to an abruptdisplacement of the worm 102 a in the meshing direction thereof, thebuffer member 106 absorbs vibrations, thereby preventing an emission ofnoises of collision. Further, the buffer member 106 is formed of theresin and is therefore effective in reducing a friction caused when theworm shaft 102 moves in the axial directions.

Moreover, in the ball bearing 103 a, the minute gap ΔS is providedbetween the inner ring and the buffer member 106 of the worm shaft 102,and hence there is absorbed a change in the axis-to-axis distancebetween the worm shaft 102 and the output shaft 105 due to the scatterin the working accuracy and to the meshing, whereby the stable operationcan be ensured.

Further, the ball bearing 103 a receives a load and a rotational torqueacting in the meshing direction of the worm 102 a, which are generatedwhen driving (when assisting) the apparatus, and controls thedisplacement of the worm 102 a. Therefore, neither a large distortionnor load occurs in the elastic body 113 of the elastic portion 103 b,thereby a durability of the elastic body 113 to be improved.

Thus, the fitting position of the bearing support portion 102 b into theelastic body 113 of the elastic portion 103 b is set eccentric, theoutside diameter of the buffer member 106 is set comparatively smallerthan the inside diameter of the inner ring of the bearing 103 a, and theelastic member 107 is provided between the worm shaft 102 and thebearing 103 c, which all cooperate to enable a pre-load mechanism to beeasily structured, the backlash to be completely eliminated, the impactto be absorbed and the tooth-butting noises (rattle noises) to berestrained.

Next, a second embodiment of the second invention of the presentinvention will be explained referring to FIGS. 14A and 14B. The secondembodiment of the second invention is substantially the same as thefirst embodiment of the second invention discussed above, wherein thesame members are marked with the same numerals or symbols, and theirrepetitive explanations are omitted. A different point is that thebuffer member 106 is fixedly fitted in the inner ring of the firstbearing 103 a. The buffer member 106 is bonded to a cylinder 115 whosewall is thin, and the cylinder 115 is fixedly fitted in the inner ring.In this construction also, the same effects as the first embodiment ofthe second invention exhibits can be expected. Further, this buffermember 106 may also be fixed to the inner ring through no intermediaryof the cylinder 115.

Subsequently, a third embodiment of the second invention of the presentinvention will be described with reference to FIGS. 15A and 15B. Thethird embodiment of the second invention is substantially the same asthe first embodiment of the second invention discussed above, whereinthe same members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that therubber-made elastic body 113 of the elastic portion 103 b is replacedwith a resin-made elastic body 116, and, as illustrated in FIG. 15B, aplurality of holes 116 a are formed along a periphery of a biasingmember 112.

In this construction, the elastic body 116 functions as an elastic bodycapable of elastically deforming in the meshing direction of the worm102 a owing to the plurality of holes 116 a. The third embodiment can bealso expected to exhibit the same effects as those in the firstembodiment of the second invention, and may require a less number ofparts, which contributes to reduce the costs.

Next, a fourth embodiment of the second invention of the presentinvention will be discussed with reference to FIGS. 16A and 16B. Thefourth embodiment of the second invention is substantially the same asthe first embodiment of the second invention discussed above, whereinthe same members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that a torsionspring 117 is adopted as a substitute for the elastic body 113 of theelastic portion 103 b. In this case, the elastic portion 103 b isconstructed of a biasing member 112 for rotatably supporting the bearingsupport portion 102 b of the worm shaft 102, the torsion spring 117wound on an outer peripheral portion of the biasing member 112, and alatching member 118 that latches both side ends of the torsion spring117 in order to support the biasing member 112 in a position eccentricin the meshing direction of the worm 102 a and is fixedly fitted in thebearing ring 114. As shown in FIG. 16B, the torsion spring 117 is openat its both side ends in an initial state but resiliently closes whenincorporated into the latching member 118, and a biasing force generatedat this time produces a pre-load force acting in the meshing directionin the worm 102 a. In this construction also, the same effects as thefirst embodiment of the second invention has can be expected.

Next, a fifth embodiment of the second invention of the presentinvention will be explained referring to FIGS. 17A and 17B. The fifthembodiment of the second invention is substantially the same as thefirst embodiment of the second invention discussed above, wherein thesame members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that thetorsion spring 117 is adopted as a substitute for the elastic body 113of the elastic portion 103 b. In this case, the elastic portion 103 b isconstructed of the biasing member 112 rotatably supporting the bearingsupport portion 102 b of the worm shaft 102, the torsion spring 117wound on the outer peripheral portion of the biasing member 112, and thelatching member 118 that latches both ends of the torsion spring 117 inorder to support the biasing member 112 in a position eccentric in themeshing direction of the worm 102 a and is fixedly fitted in the bearingring 114. The biasing member 112 is fitted at the its center thereof onthe worm shaft 102 b and generates a pre-load with respect to the worm102 a by dint of a rewinding force of the torsion spring 117 wound onalong the outer periphery thereof concentrically with a hole thereof. Acontact portion between the biasing member 112 and the torsion spring117 is opposed to the wheel 104 and is well short for an inner peripheryof the torsion spring 117, the structure being such that the pre-loadgenerated by the torsion spring 117 can be efficiently transferred tothe biasing member 112. Further, the latching member 118 is fixedlyfitted in the housing 101 and fixed by the bearing ring 114. Thelatching member 118 may be formed either by press working or of a resin,etc..

As illustrated in FIG. 17B, the torsion spring 117 has two pieces ofhooks 117 a provided at both ends and disposed respectively in positionsthat are shifted by 1800 in phase with respect to a winding center ofthe torsion spring 117 in the initial state, wherein these hooks 117 aare latched by projections of the latching member 118. Moreover, thetorsion spring 117 is given a torsional torque beforehand. With thiscontrivance, the biasing member 112 is, before assembling the worm 102a, held by the projections 118 a of the latching members in positionsslightly shifted with respect to the elastic portion, and the biasingmember 112 is displaced by assembling the worm 102 a with the resultthat the pre-load force is to be produced for the worm 102 a. Therefore,any processes specializing in adjusting and in giving the pre-load arenot required, and a built-in characteristic is improved. Moreover, thepredetermined pre-load force can be ensured even by setting the springconstant comparatively low. In this construction also, the same effectsas those in the first embodiment of the second invention can beexpected.

Moreover, a sixth embodiment of the second invention of the presentinvention will be explained with reference to FIGS. 18A and 18B. Thesixth embodiment of the second invention is substantially the same asthe first embodiment of the second invention discussed above, whereinthe same members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that a spiralspring 119 is adopted as a substitute for the elastic body 113 of theelastic portion 103 b. One end of the spiral spring 119 is fixed to theouter peripheral portion of the biasing member 112 rotatably supportingthe bearing support portion 102 b of the worm shaft 102, and the otherend thereof is fixed to the bearing ring 114. The biasing member 112 iseccentric in the meshing direction of the worm 102 a with respect to theaxis of the bearing ring 114, and, when assembling the worm shaft 102into the ball bearing 103 a, the spiral spring 119 deforms to producethe pre-load acting in the meshing direction in the worm 102 a. In thisconstruction also, same effects as those in the first embodiment of thesecond invention can be expected.

INDUSTRIAL APPLICABILITY

As described above, according to the first invention of the presentinvention, the motor is installed with respect to the output shaft insuch a position that the length which is the sum of the working radiusof the worm and the working radius of the worm wheel becomes theaxis-to-axis distance between the output shaft and the motor shaft. Thebearing is installed with respect to the output shaft in such a positionthat the axis-to-axis distance between the shaft support hole and theoutput shaft becomes slightly smaller than the axis-to-axis distancebetween the output shaft and the motor shaft. The elastic member havingthe predetermined elasticity and thickness is interposed between theinner peripheral surface of the shaft support hole of the bearing andthe rotary shaft. The outside diameter of the portion of the rotaryshaft on which the elastic member is fitted, is set smaller than theinside diameter of the shaft support hole to such a degree that theelastic member gives the proper biasing force towards the worm wheel ofthe worm. Therefore, with the simple construction, the backlash can beeliminated, and the tooth-butting noises can be reduced without anydecline of the transmitting performance of the auxiliary steering force.

Further, the pre-load force can be adjusted based on the material andthe configuration of the elastic member, and hence the dimensionalaccuracies are not excessively required of the rotary shaft and thebearing as well, thereby facilitating the dimensional control.

Moreover, according to the second invention of the present invention,with respect to the motor-sided bearing, the elastic members aredisposed on both sides in the axial direction thereof and adjacentlythereto, and the worm is set slightly movable in the axial directions.Besides, the bearing on the distal side from the motor is the rollingbearing, and the elastic portion for biasing the worm in the meshingdirection is provided on the rotary shaft. The outer ring of the bearingis fixedly fitted in the cylindrical bearing support member fixedlyfitted in the housing, and the inner ring of this bearing looselyreceives therein the cylindrical buffer member fixedly fitted on therotary shaft. The elastic portion is constructed of the biasing memberfor rotatably supporting the rotary shaft and of the elastic bodyreceiving therein this biasing member in the position that is eccentricin the meshing direction of the worm with respect to the axis of thebearing, this elastic body being fixed in the vicinity of the bearingsupport member. Therefore, the worm has the pre-load force produced inthe meshing direction thereof, and, with the simple construction, thebacklash can be eliminated completely.

Particularly, the rate at which the volume of the elastic body occupiesthe elastic portion is taken large, whereby the initial eccentricquantity needed for generating the pre-load can be taken large and thespring constant of the elastic body can be decreased. Therefore, evenwhen the configuration of the worm might change due to the scatter inworking accuracy and the abrasion of the gear, it is feasible to stablymaintain the fixed pre-load force and to effectively prevent thetooth-butting noises of the gears.

Further, when the force is applied to the rotary shaft, the rotary shaftmoves in the axial directions within the limit of the elasticity of theelastic member, with the result that the worm and the gear portion ofthe worm wheel mesh with each other in the proper positions to absorbthe impact. It is therefore possible to reduce the tooth-butting noiseswithout any decline of the transmission performance of the auxiliarysteering force.

Moreover, the bearing receives the load and the rotational torque actingin the meshing direction of the worm, which are generated when drivingthe apparatus, thus controlling the displacement of the worm. Therefore,neither the large distortion nor load occurs in the elastic body of theelastic portion, which leads to an improvement of a lifetime of theelastic body.

Furthermore, according to the third invention of the present invention,the electric power steering apparatus includes the housing, the motorattached to this housing and transmitting the auxiliary steering forceto the rotary shaft through the motor shaft, the worm formed on orfitted on this rotary shaft and having the gear portion formed of themetal or the resin, the rolling bearings provided in the housing,disposed respectively in the positions on both sides of the worm androtatably supporting the rotary shaft, the output shaft transmitting thesteering force for steering an axle and rotatably supported in thepredetermined position of the housing, and the worm wheel formed on orfitted on the output shaft in a way that meshes with the worm and havingthe gear portion formed of the resin, wherein the pre-load mechanism forapplying the pre-load acting towards the worm wheel is provided at theshaft side end portion, distal from the motor, of the rotary shaft. Itis possible to provide the thus simply-constructed electric powersteering apparatus capable of eliminating an existence of the backlashand reducing the tooth-butting noises without any decline of the powertransmitting performance.

1-14. (canceled)
 15. An electric power steering apparatus comprising: ahousing; a motor attached to said housing and transmitting an auxiliarysteering force to a rotary shaft through a motor shaft; a worm formed onor fitted on said rotary shaft and having a gear portion formed of ametal or a resin; bearings provided in said housing, disposedrespectively in positions on both sides of said worm and rotatablysupporting said rotary shaft, said bearings including a motor-sidebearing and a second bearing which is positioned away from said motor;an output shaft transmitting a steering force for steering an axle androtatably supported in a predetermined position of said housing; and aworm wheel formed on or fitted on said output shaft in a way that mesheswith said worm and having a gear portion formed of a resin, said motorincluding a worm gear mechanism installed in such a position that alength which is the sum of a working radius of said worm and a workingradius of said worm wheel becomes an axis-to-axis distance between saidoutput shaft and said motor shaft, wherein elastic members are disposedon both sides, axially, of said motor-side bearing and adjacent to saidmotor-side bearing, said rotary shaft being set slightly axiallymovable, said second bearing is a rolling bearing, an elastic portionbeing provided on said rotary shaft and biasing said worm in a meshingdirection in parallel with said rolling bearing, an outer ring of saidrolling bearing is fixedly fitted in a cylindrical bearing holdingmember fixedly fitted in said housing, an inner ring of said rollingbearing loosely receiving therein a cylindrical buffer member fixedlyfitted on said rotary shaft, said elastic portion includes a biasingmember rotatably supporting said rotary shaft and an elastic bodyreceiving therein said biasing member in a position eccentric in themeshing direction of said worm with respect to an axis of said rollingbearing, and fixed in a vicinity of said bearing holding member, whereinat least a half, in an axial direction, of a spline connecting portionbetween said rotary shaft and said motor shaft is so set as to bepositioned within a shaft support hole of said motor-side bearing. 16.An electric power staring apparatus according to claim 15, wherein anelastic member associated with said motor-side bearing, or said rotaryshaft in the vicinity of the second bearing, or both, is or are providedwith a stopper for controlling a quantity of axial movement of saidrotary shaft.
 17. An electric power staring apparatus according to claim15, wherein said rotary shaft and said motor shaft are spline-connectedto each other, and said rotary shaft is so formed as to be swayableabout the spline connecting portion serving as a fulcrum.
 18. Anelectric power staring apparatus according to claim 17, wherein saidmotor shaft is formed with a male spline portion, said rotary shaft isformed with a female spline portion, and said male spline portion isformed in a drum-like shape so that its diameter is small at both sideends thereof in axial directions and is large at a central portionthereof.